This invention relates to a degaussing circuit of the type commonly used in color television sets and color monitors. More particularly, this invention relates to an improved degaussing circuit having a degaussing coil and a thermistor element with a positive temperature coefficient for its resistance (or a PTC element) connected in series.
FIG. 4 is a circuit diagram of a power source part 31 and its adjoining parts of a prior art color television set, showing a power source 32 of frequency 50 or 60 Hz. The power source part 31 includes a main switch 33 connected in series with the power source 32, a rectifying circuit 34 which may comprise a diode and a smoothing circuit 35 with a capacitor. The main circuit 36 of a color television set may be connected to this power source part 31 such that power is supplied from the latter to the former.
A degaussing circuit 37 for degaussing (or demagnetizing) the cathode ray tube of the color television set is also connected to the power source part 31. The degaussing circuit 37 includes a PTC element 38 and a degaussing coil 39 which are connected to each other in series. A relay 40 for switching the degaussing circuit 37 on and off is connected, and there is also provided a driver circuit 41 for supplying HIGH and LOW voltage signals for switching the relay 40 on and off. The driver circuit 41 includes a transistor 41a serving as a switching element connected between a power source voltage terminal +V.sub.cc and the ground potential. The HIGH and LOW voltage signals are applied to the base terminal of the transistor 41a through a resistor 41b so as to switch the relay 40 on and off. In FIG. 4, numeral 31a indicates a fixed resistor for attenuating the rush current when the power source part 31 is switched on. This fixed resistor 31a may be replaced by an NTC element.
Operations of the degaussing circuit 37 shown in FIG. 4 will be described next. In the beginning, since the relay 40 is in the closed condition, a current corresponding to the frequency of the power source 32 flows into the degaussing circuit 37 when the main switch 33 of the color television set is switched on, and this is transformed by the PTC element 38 into an alternating decay current with the same frequency. An alternating decay magnetic field corresponding to this alternating decay current is generated by the degaussing coil 39 and the cathode ray tube disposed close by is thereby degaussed, or demagnetized.
This act of degaussing is completed within a few seconds after the main switch 33 is switched on. Thus, the degaussing circuit 37 may be switched off by activating the driver circuit 41 through a time constant circuit (not shown) to open the relay 40 and to stop the current to the degaussing circuit 37. Thereafter, the degaussing circuit 37 will not be activated until the main switch 33 is switched on again.
The aforementioned alternating decay current which flows through the degaussing coil 39 is as shown in FIG. 5, its amplitude decreasing with time but its frequency remaining the same as that of the commercial power source supplied from the power source 32, that is, 50 Hz or 60 Hz. If the amplitude differential P.sub.max, or the difference between the peak amplitude in one period of the alternating current and that of the next period (as shown in FIG. 5), is too large, the cathode ray tube may end up being magnetized instead such that not only does it fail to be demagnetized but an unevenness in color may be generated on the screen.
One of the attempts to prevent such an occurrence has been to use a larger PTC element 38 so as to reduce the magnitude of the amplitude differential P.sub.max. In order to make P.sub.max smaller, it is necessary to attenuate the current more slowly. This may be accomplished by slowing down the increase in the resistance of the PTC element (or its temperature) and it has been known to use a larger element with a larger thermal capacity for this purpose. If a larger PTC element 38 is used, however, the degaussing circuit 37 cannot be miniaturized and hence the portion of the color television set which incorporates it also cannot be made compact.